Reactivating First (and Only) Instance of an Application

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

Sometimes restricting an application to a single instance is desirable. In that case, when the user starts a second instance of the application, a message box that says that the application is already running can be displayed. A more user-friendly approach is to bring the first instance to the front and activate it.

To accomplish this, perform the following four steps:

1. Obtain the main window handle of the first instance.

2. If the first instance has an open dialog box, obtain the window handle of its last active pop-up window.

3. Bring the parent and pop-up window (if present) to the front.

4. Do not run the second instance.

MORE INFORMATION

To determine whether an instance of the application is already running, the first method below uses the FindWindow function to look for the an application with the same main window class name. The second method uses the hPrevInstance parameter to the WinMain function.

The FindWindow method does not depend on any memory architecture or on the hPrevInstance parameter; therefore, it is the suggested method for future portability. The hPrevInstance assumes a memory architecture for the operating system that allows a task to access another application's data segments. In future versions of Windows, this may not be possible if applications do not share the same local descriptor table (LDT).

Method 1: Use FindWindow Function

HWND FirsthWnd, FirstChildhWnd;

if (FirsthWnd = FindWindow("MyMainWindowClassName", NULL))

   {
   // Found another running application with the same class name.
   // Therefore, one instance is already running.
   // NOTE: "MyMainWindowClassName" must match the class name of the
   // program's main window. In the GENERIC sample application,
   // provided with the Microsoft Windows Software Development Kit,
   // versions 3.0 and 3.1, the chass name is "GenericWClass", which
   // is used in the InitApplication and InitInstance functions.

   FirstChildhWnd = GetLastActivePopup(FirsthWnd);
   BringWindowToTop(FirsthWnd);         // bring main window to top

   if (FirsthWnd != FirstChildhWnd)
      BringWindowToTop(FirstChildhWnd); // a pop-up window is active
                                        // bring it to the top too

   return FALSE;                        // do not run second instance
   }

Method 2: Use hPrevInstance Parameter

// Declare a global variable to save the handle of the first instance
// of the main window.

// Make the following modifications to the WinMain function:

HWND FirstChildhWnd;    // handle of last active pop-up window of the
                        // first application instance

if (!hPrevInstance)     // other instances of application running?
   if (!InitApplication(hInstance))  // initialize shared things
         return FALSE;  // exits if unable to initialize
   else {}

      // retrieve the main window handle from the first instance
   {
   GetInstanceData(hPrevInstance, (NPSTR)&FirsthWnd, 2);

   FirstChildhWnd = GetLastActivePopup(FirsthWnd);
   BringWindowToTop(FirsthWnd);         // bring main window to top

   if (FirsthWnd != FirstChildhWnd)
      BringWindowToTop(FirstChildhWnd); // a pop-up window is active
                                        // bring it to the top too

   return FALSE;                        // do not run second instance
   }


// Add this line to the InitApplication function:

// Add these two lines to the InitInstance function after the
// "hWnd = CreateWindow(...);" call:

if (!FirsthWnd)       // If this is the first instance,
   FirsthWnd = hWnd;  // save the window handle.